Friday, January 28, 2005

Chemistry: Acidic: 3

3. Acids occur in many foods, drinks and even within our stomachs

Define acids as proton donors and describe the ionisation of acids in waterAn acid is a proton donor. When an acid molecule is in contact with water it can ionise, donating a proton to a water molecule.A hydrogen atom, H, consists of one proton and one electron. A hydrogen ion, H+ , is formed when a H loses its electron, leaving just a proton. A proton and a hydrogen ion are thus the same and can be represented by H+.When an acid molecule is placed in water, it can ionise, releasing a proton and forming a negative ion. The proton, H+, can attach to a water molecule, H2O,forming what is called a hydrated hydrogen ion or hydronium ion, H3O+.

Sulfuric acid is called a diprotic acid because each molecule can release up to two protons.

Phosphoric acid is called a triprotic acid because each molecule can release up to three protons.

Identify acids including acetic (ethanoic), citric (2-hydroxypropane-1,2,3-tricarboxylic), hydrochloric and sulfuric acid- Acetic acid (ethanoic acid)occurs naturally in the decomposition of biological material, such as oxidation of wine alcohol, but most acetic acid used by humanity is manufactured industrially.CH3COOH:

- Citric acid occurs naturally in fruits but most that is added to food as preservatives is manufactured.2-hydroxypropane-1,2,3-tricarboxylic:

- Hydrochloric acid is naturally secreted in the stomach as part of the digestive juices, but is also industrially produced on a large scale.HCl:

- Sulfuric acid is mostly manufactured, but it can also occur naturally. For example, most sulfur dioxide released into the earth’s atmosphere is oxidised and dissolved in water to form the sulfuric acid in acid rain. If the acid rain results from volcanic eruption it could be regarded as natural, but if acid rain results from smelting of sulfide ores, it could be regarded as manufactured.H2SO4:

Describe the use of the pH scale in comparing acids and basesThe pH scale is used to compare the concentration of hydrogen ions in solutions of acids and bases, in terms of log10.H2O(l) + H2O(l) OH-(aq) +H3O+(aq) orH2O(l) OH-(aq) +H+(aq)In acidic solutions, there are more ionised hydrogen ions in the water, and in a basic solution, there are more OH- ions.Note that:The [H+] x [OH-] is the same for all aqueous (water) solutions. It does not matter whether the solution is of an acid, a base, a salt or a mixture of these.[H+] x [OH-] is the ionisation constant for water, Kw = 1.0 x 10-14 at 25oC.In pure water without any dissolved gas, [H+] = [OH-] = 10-7 mol L-1 and so pH =7.In an acidic solution, [H+] > 10-7 mol L-1 and pH <> 7.

Describe acids and their solutions with the appropriate use of the terms strong, weak, concentrated an diluteA concentrated solution contains a large amount of solute in a given amount of solution. A 10 mol L-1 solution would be called concentrated.A dilute solution contains a small amount of solute in a given amount of solution. A 0.01 mol L-1 solution would be called dilute.A strong acid is one that fully ionises, eg. HCl. (at same conc. a strong acid has more H+ ions than a weak)A weak acid is one that does not fully ionise in solution, eg. citric acid.

Identify pH as –log10[H+] and explain that a change in pH of 1 means a ten-fold change in [H+]In a neutral solution at 25°C, the concentration of the hydronium (or hydrogen ions has a value of 1.0 x 10-7 mol L-1, which equals the concentration of the hydroxide ions, that is [H+] = 1.0 x 10-7 mol L-1 = [OH-]. Describing he acidity of a solution using with negative powers can be cumbersome, so a more convenient approach is to use pH to describe acidity levels. PH is defined as the negative of the logarithm (to the base 10) of the hydronium/hydrogen ion concentration, and it is a unitless number. Because it is a log to the base ten, there is a ten-fold change in the number of hydrogen ions every time there is a change of 1 pH.

Compare the relative strengths of equal concentrations of citric, acetic and hydrochloric acid and explain in terms of the degree of ionisation of their moleculesHydrochloric acid is a strong acid, close to 100% ionised in solution (theoretical pH = 1.0)

Describe the difference between a strong and a weal acid in terms of an equilibrium between the intact molecule and its ionsA strong acid is one in which the molecules present practically all ionise when placed in water, e.g. hydrochloric acid, HCl.

A weak acid is one where only a small proportion of the molecules ionise, e.g. acetic acid (ethanoic acid), CH3COOH. Here, an equilibrium is created.

If 0.1 M solutions of each are prepared it should be seen that hydrochloric acid has a pH of 1 corresponding to a [H+] = 10-1 = 0.1 M. That is, practically every HCl molecule has ionised, producing a H+.By contrast a 0.1 M solution of acetic acid will have a pH close to 3 indicating a [H+] close to 10-3 = 0.001 M. Only about 0.001 / 0.1 = 1% of the acetic acid molecules have ionised producing a H+.

Solve problems and perform a first-hand investigation to use pH meters/probes and indicators to distinguish between acidic, basic and neutral chemicalsUsing a pH meter or probe is a non-destructive way of testing whether a chemical solution is acidic, basic or neutral. Provided the pH meter electrode or probe is washed well with distilled water between measurements, the solutions tested should be unaffected.Using indicator solution or indicator paper is a destructive way of testing, as the indicator will contaminate the portion of solution tested.Before performing the investigation, calibrate the pH meter electrode or data logger probe before use. The electrode or probe should be placed in solutions of known pH before use to test the required acidic, basic and neutral solutions and the meter or data logger adjusted to give appropriate readings.

Plan and perform a first-hand investigation to measure the pH of identical concentrations of strong and weak acids- Conductivity test: a strong acid will conduct more since there are more ionized molecules- Indicators: strong acids will be more acidic- pH probe

Gather and process information from secondary sources to write ionic equations to represent the ionisation of acidIf an acid is a strong acid, the equation will usually be written with an arrow, , from left to right showing that ionisation of the acid molecules is almost complete.If an acid is a weak acid, the equation will usually be written with the reversible arrows, , that show that significant amounts of reactants (un-ionised molecules) as well as products ( H+ and an acid anion) are present in equilibrium.

Use available evidence to model the molecular nature of acids and simulate the ionization of strong and weak acids

Ionisation of two acids of the same concentration in water.

Gather and process information from secondary sources to explain the use of acids as food additivesA food additive is defined as “any substance not normally consumed as a food in itself and not normally used as a characteristic ingredient of food whether or not it has nutritive value, the intentional addition of which to food for a technological purpose in the manufacture, processing, preparation, treatment, packaging, transport or storage of such food results, or may be reasonably expected to result, in it or its by-products becoming directly or indirectly a component of such foods”. Foods are subjected to many environmental conditions, such as temperature changes, oxidation and exposure to microbes, which can change their original composition. Food additives play a key role in maintaining the food qualities and characteristics that consumers demand, keeping food safe, wholesome and appealing from farm to fork.Food acids, especially vinegar and citric acid, are added to make the flavour of foods "sharper", and also act as preservatives and antioxidants, also to provide leavening or control acidity/ alkalinity or to improve or maintain nutritional value.Leavening agents that release acids when heated can react with baking soda to help cakes, biscuits and other goods to rise during baking. Vitamins and minerals are added to many common foods such as milk, flour, cereal and margarine to make up for those likely to be lacking in a person's diet or lost in processing, eg. ascorbic acid (vitamin C).Ascorbic acid, propionic acid and citric acid are used to maintain palatability (taste); citric acid, fumaric acid, phosphoric acid and lactic acid are used to control acidity/alkalinity.Ascorbic acid is an antioxidation thus is added to food to prevent spoilage by oxidation.

Identify data, gather and process information from secondary sources to identify examples of naturally occurring acids and bases and their chemical composition

Process information from secondary sources to calculate pH of strong acids given appropriate hydrogen ion concentrations1. Calculate concentration of strong acid/base (c= moles/volume)2. Write equation showing the number of moles of H+ or OH- formed per mole of acid3. Multiply this number by the original concentration of the acid/base4. In bases, determine [H+] by: [H+] = 10-14/[OH-]5. Calculate pH value: pH = -log[H+]